Further studies have confirmed our recent report that the thyroid hormones, and reverse T3(rT3), bind to the 3 major lipoprotein classes. They now show a greater overall binding of T3 compared to T4, which is greater than rT3; i.e., the order of their biological potency. Displacement studies with iodothyronines and drugs indicate that at least part of this binding is the result of interaction between thyroid hormones and the apolipoprotein moieties.